Conventionally, compositions containing tin can be used in rubber compositions for tires and the like (see, e.g., Japanese Unexamined Patent Application Publication No. 2011-57946A, Japanese Patent No. 3913346, or International Patent Publication No. WO/2007/119675).
However, when a rubber composition contains a divalent tin carboxylate like tin bis(2-ethylhexanoate), a tetraalkyl tin (having tetravalent tin), or a dialkyl tin dihalide (having tetravalent tin), the viscosity of the unvulcanized rubber is high, and rubber properties like tensile strength and elongation at break are poor.